Carbonless copy paper is coated with ink containing microcapsules which respond to a sharp impact from a typewriter or other writing instrument to produce a duplicate image on a copy sheet disposed under the top sheet. The ink microcapsules of the coating are protected from premature smudging or rupture by means of starch particles included in the paper coating interspersed with the ink microcapsules. The starch particles do not crush under the normal paper handling procedures to prevent premature rupture of the microcapsules, but do not prevent imaging due to typewriter impact pressure or from a writing instrument. British patent specification No. 1,252,858 published Nov. 10, 1971, and British Pat. No. 1,232,347 published May 19, 1971 describe ink microcapsule/protective starch material coatings of this general type. These references rate arrowroot starch particles as the most functional of the various starch particles tested.
U.S. Pat. Nos. 3,876,629, 3,901,725 and 3,951,948 are directed to special large granule starch particles which are obtained from a bimodal cereal starch such as wheat, barley or rye by the processes described. These special large granule starches replace the more expensive and scarce arrowroot starch.
U.S. Pat. Nos. 3,996,060 and 3,996,061 are directed to a protective material which comprises refined, large granule starch derived from legume starches from which non-starch materials have been removed.
Although the first series of patents mentioned describe a method of wet separation of the large granule portion of starch particles from a prime grade, bimodal wheat starch, the yield of large granule starch product useful for carbonless copy paper coatings was about 15-20% of the total weight of the prime grade wheat starch supplied to the separation equipment. The by-product streams from the separation process described could only be sold in competition with ordinary starch, and therefore, at much lower prices than the large granule product used in the carbonless copy paper coatings.
The second series of patents mentioned above describe legume starch particles which are presently obtained in an unrefined form from Canada, and then treated to remove non-starch material including protein and fiber. The legume starches include faba bean starch and yellow field pea starch, which may have a particle size distribution in the range of 25-50 microns and 20-40 microns, respectively. The refined legume starch particles have proven to be very satisfactory protective particles in carbonless copy paper coatings, and they are more readily available then arrowroot.
The need for improving the yield of usable protective material derived from wheat starch becomes evident when the selling price of the by-product small granule portion is compared to the much greater value of the large granule protective material which is used in carbonless copy paper coatings. Several other important economic considerations are the termination of the Federal government incentive program for developing industrial uses for wheat starch, coupled with the much more stringent effluent limitations now being established at most starch milling locations. Faced with these serious additional cost burdens, it became necessary to discover how to use different starting materials, and to improve the yield of useful protective starch particles in proportion to by-product. Several approaches were tried without success. One attempted approach was to chemically increase the size of the substandard size starch particles, to thereby increase the yield. Several recirculation systems were also tried, without significant success, until the recirculation system of the subject invention was conceived and tried. It has proved to be effective on a commercial scale to increase the yield of usable protective starch particles from about 15-20% by weight of feed stream up to more than 40 % by weight of the feed stream.